1. Field of the Invention
This invention relates generally to high temperature cements and more particularly to hydrothermal cements containing a diopside or serpentine phase.
2. Description of the Prior Art
Hydrothermal curing refers to treatment of cement at elevated temperatures and pressures (i.e., above ambient) in the presence of water. Hydrothermally cured cements designed especially for use in oil and gas wells have heretofore been operable at temperatures up to about 320.degree. C. at a pressure of 20,000 psi.
Most hydrothermal cements are calcium silicate based cements, such as portland cement. The most important components of these cements are belite (dicalcium silicate, C.sub.2 S) and alite (tricalcium silicate, C.sub.3 S). When cured hydrothermally, C.sub.2 S and C.sub.3 S present in these cements react to form various semicrystalline and crystalline calcium silicates and calcium silicate hydrates, the structures of which depend on temperature and pressure. Typical of the hydrates which form are .alpha.-C.sub.2 SH, .beta.-C.sub.2 SH (hillebrandite) and C.sub.5 S.sub.2 H (calciochrondite).
When extra silica is added to cements containing C.sub.2 S and C.sub.3 S, CS (wollastonite) and various calcium silicate hydrates are formed depending on the stoichiometric composition of the starting materials and the curing temperature and pressure used. Typical of these latter hydrates are foshagite, (Ca.sub.4 Si.sub.3 O.sub.9 (OH).sub.2) xonotlite, (Ca.sub.6 Si.sub.6 O.sub.17 (OH).sub.2) tobermorite, (Ca.sub.5 (Si.sub.6 O.sub.18 H.sub.2.4H.sub.2 O)) and truscottite (Ca.sub.6 Si.sub.10 O.sub.24 (OH).sub.2). These compounds may undergo further changes with time such as conversion to more stable phases.
Hydrothermal cements are often used in wells to bind steel casings to the wall rock of bore-holes, to seal porous formations, and to plug wells in order to prevent contamination of the environment and migration and loss of valuable resources.
Cements used in deep oil and hydrothermal wells must meet stringent requirements of liquid and gas permeabilities, stability, strength, setting time and pumpability due to the conditions of high temperature, high pressure and the highly corrosive environment existing in such wells. The calcium silicate-based hydrothermal cements heretofore used, however, have not proven to be completely satisfactory in that they tend to deteriorate in the geothermal environments found in hot, corrosive wells, often causing thereby failure of the wells. While it is not completely known why this deterioration occurs, it is usually attributed to such factors as unacceptably high porosity and permeability, structural failure due to phase changes in the cement, or fracture induced by thermal shock at high temperatures. Thus, calcium silicate-based cements, which will more satisfactorily meet the stringent requirements necessary for use in very deep oil wells or in hydrothermal wells, would be quite advantageous.
Attempts have been made to modify calcium silicate-based cements, such as portland cement, in order to improve their properties. One such attempt has been to add magnesia or another magnesium source, such as serpentine, to the portland cement. Serpentine, Mg.sub.3 Si.sub.2 O.sub.5 (OH).sub.4, occurs in three polymorphs: chrysotile (i.e., asbestos), antigorite and lizardite. Each polymorph contains numerous polytypes.
One such modification of portland type cements, described in Ulfstadt, U.S. Pat. No. 2,880,101, Hayes, U.S. Pat. No. 3,582,277 and Collepardi, U.S. Pat. No. 4,046,583, is the addition of magnesia, followed by curing at temperatures up to about 200.degree. C. Under the conditions as described in these prior art references, the magnesia does not react with the portland cement, but is merely slaked according to the reaction MgO+H.sub.2 O.dbd.Mg(OH).sub.2 to cause swelling. Excess unreacted magnesia is undesirable in well cement since the above reaction may cause uncontrollable expansion and consequent disruption of the cement structure.
Moreover, the addition of water to cement containing magnesia results in a slurry of too high a viscosity and, consequently, reduced pumpability, which limits the range of applicability of such cements. While the high viscosity can be overcome by addition of large amounts of water to the slurry, excess water has an undesirable effect on the setting, curing and physical properties of the cement.
It is also known to add asbestos fibers to cement followed by heating the mixture to temperatures up to 200.degree. C. Under these conditions, the asbestos is not calcined and remains as serpentine in fibrous form. An example of this type of asbestos modified cement is described in Schulze, U.S. Pat. No. 3,880,664. Asbestos fibers do not readily react with compounds found in portland cement when cured hydrothermally between 200.degree. and 440.degree. C. and pressures up to 30,000 psi. Slurries containing asbestos fibers also require very high water to solids ratios. Therefore, formulations prepared for deep well cementing can only tolerate small amounts of asbestos fibers and still maintain acceptable workability (viscosity and pumpability). In addition, the presence of more than about 2% asbestos fibers in cement causes a reduction in compressive strength.
An attempt has also been made to make a hydraulic cement using only calcined serpentine rock. The cement obtained, however, was found to have low compressive strength following curing at 18.degree. C. and, therefore, to be of only slight commercial value. (H. G. Midgley, Cement and Concrete Research, 9, 157 (1979)).
In April 1976, the ERDA Division of Geothermal Energy initiated a program for the development of improved cements specifically designed for geothermal well applications. The motivation for the program stems from an assessment that the cements currently used deteriorate in the geothermal environments, and that the life expectancy of a geothermal well and, therefore, the economics of geothermal power can be improved significantly if better materials are developed.
The cements used to complete geothermal wells are similar to those used in oil and gas wells. These materials deteriorate in geothermal environments and the failure of several wells has been attributed to cement degradation. Total loss of a well can result in a loss of up to $1,000,000. A partial well failure due to inadequate cement can result in an energy production decline or environmental damage. Thus, a real economic need exists for the development of improved cementing materials.
For use in geothermal wells, cementing materials with the following characteristics are needed:
1. Twenty-four hour compressive strength of at least 1000 psi.
2. Water permeability less than 0.1 millidarcy.
3. Cement/steel bond strength of at least 10 psi.
4. Stability for at least 96 days in hydrothermal water or 20% brine (i.e., no significant reduction in strength or increase in permeability after exposure at 400.degree. C.).
5. Non-corrosive to steel.
6. Set time of 0.5-6 hour.